[mothered]

Will the phone get blacklisted?

It's not possible to be conclusive.

What is the best or most effective way to do this. Very old account, double digit age, no refunds ever and thousands of dollars of spend

A mature account In good standing, Is less likely to raise suspicion.

My friend is wanting to do the empty box method. Would this work?

What's the precise weight of the phone?

Potentially purchase another item at the same time (also FBA) to have the choice to do partial.

As per above, It depends on the weight.